Wideband wireless base station making use of time division multiple access bus having selectable number of time slots and frame synchronization to support different modulation standards

ABSTRACT

A wireless communication system basestation making use of a wideband, multichannel digital transceiver having incorporated therein a time division multiple-access (TDM) bus for providing digital samples of a plurality of wireless communication channels, wherein the time slot duration and frame rate of the TDM bus may be reconfigured. The invention allows various air interface standards, even those having different channel bandwidths, to be serviced by the same basestation, without having to install additional or different equipment, and by automatically redistributing signal processing resources, eliminating the need to reconfigure the basestation when different types of wireless signaling must be accommodated.

This is a continuation of application Ser. No. 08/402,585 filed on Mar.13, 1995 now U.S. Pat. No. 5,592,480.

FIELD OF THE INVENTION

This invention relates generally to communication networks, and inparticular to a wireless communication system basestation making use ofa wideband, multichannel digital transceiver having incorporated thereina time division multiple-access (TDM) bus for providing digital samplesof a plurality of wireless communication channels, wherein the TDM bushas a selectable number of time slots per frame, and a selectable framesynchronization rate, to permit dynamic allocation of modulator anddemodulator signal processing resources, and to support wirelessmodulation standards of different bandwidths.

BACKGROUND

The basestations used by the providers of current day multiple channelwireless communication services, such as cellular mobile telephone (CMT)and personal communication systems (PCS), typically designate signalprocessing equipment for each single receiver channel. This is probablya result of the fact that each basestation is configured to providecommunication capability for only a limited predetermined number ofchannels in the overall frequency spectrum that is available to theservice provider.

A typical basestation may thus contain several racks of equipment whichhouse multiple sets of receiver and transmitter signal processingcomponents that service a prescribed subset of the available channels.For example, in an Advanced Mobile Phone Service (AMPS) cellular system,a typical basestation may service a pre-selected number of channels,such as 48, of the total number, such as 416, of the channels availableto the service provider.

Certain types of wireless service providers would prefer, however, toemploy equipment that would be more flexible, both in terms of where itcan be located, as well as in the extent of the available bandwidthcoverage provided by a particular transceiver site. This is particularlytrue in rural areas where cellular coverage may be concentrated along ahighway, and for which the limited capacity of a conventional 48 channeltransceiver may be inadequate. This may also be true in other instances,where relatively large, secure, and protective structures for multipleracks of equipment are not necessarily available or cost effective, suchas for PCS applications.

One way to resolve this difficulty is to implement a basestationtransceiver using a high speed analog-to-digital (A/D) converter andequipment which makes use of efficient digital filtering algorithms suchas the Fast Fourier Transform (FFT), to separate the incoming signalenergy into multiple ones of the desired channels. On the transmit side,this basestation implementation includes an inverse FFT processingcombiner which outputs a combined signal representative of the contentsof the communication channel signals processed thereby. In this manner,relatively compact, lightweight, inexpensive, and reliable digitalintegrated circuits may be used to cover the entire channel capacityoffered by the service provider, rather than only the subset of theavailable channels.

Thus unlike prior art basestations, the wideband digital basestation iscapable of receiving any channel. While this provides a certain numberof advantages as described above, it also poses a number of uniqueproblems to the service provider.

Perhaps most importantly, there exists a need to efficiently support avarying number of active channels and the required connections into thepublic switched telephone network.

These connections should be made in such a way as to simplify callcontrol. Indeed, it would be desirable for as many of the call set upcontrol functions required by such a basestation were handled to themaximum extent possible by the basestation itself.

By so simplifying the network interface, the Mobile Telephone SwitchingOffice (MTSO) and/or Mobile Switching Center (MSC) through which thebasestation is connected to the Public Switched Telephone Network (PSTN)may be freed, as much as possible, from the details of maintaining aproper connection from the PSTN to the remote subscriber unit.

Secondly, the basestation should make efficient use of the availableresources to process each call. In particular, while the widebandchannelizer separates the signals into channels, certain other signalprocessing resources such as demodulators and modulators are alsorequired.

Using the wideband front end, any channel in the bandwidth available tothe service provider is available at any time. However, it is desirablefor such a basestation to only activate as many of the other,per-channel resources as is required to support the present calldensity.

By making the basestation's implementation of call processing resourcesas modular as possible, the basestation could initially be configured tosupport a limited number of channels. Then, as the demand for servicesgrows, additional channels could be supported by the addition of thenecessary resources.

In other instances, the basestation should be reconfigurable in theevent of an change or expansion in one type of service. For example,given the emergence of several air interface standards such as codedivision multiple access (CDMA) as well as time division multiple access(TDMA) standards for cellular, it is desirable for a given widebandbasestation to be able to support each such standard, thereby reducingthe number of such basestations that need to be deployed. However, itwould be desirable if the resources allocated to one particular airinterface, when no longer needed, could then be made available toprocess signals formatted using the other air interface. That is, as thedemands of one type of service or the other come and go, the basestationshould be automatically reconfigured, without requiring an investment innew or different basestation resources.

Thus, several difficulties exist with a wideband digital basestationthat can process at any time, any one of many channels in the RFbandwidth available to a service provider.

SUMMARY OF THE INVENTION

Briefly, the invention is a wideband transceiver basestation for awireless communication system. The receiver portion of the basestationincludes a digital channelizer which provides digital samples ofmultiple wireless channel signals, and a time division multiplexed (TDM)data bus, to provide switching functionality between the various channeloutputs and other basestation receiver resources such as digitaldemodulators.

On the transmitter side, basestation signal processing resources such asdigital modulators are also connected to a multichannel digital combinerover the TDM bus. Thus, the same flexibility in switching functionalityis provided between transmitter signal processing resources and thetransmitter channel inputs.

A synchronization and clock generation circuit has the capability ofselecting the number of time slots, as well as the bus frame rate, to beused on the TDM bus. The number of time slots and the bus frame ratedepend upon, respectively, the number of the channel signals and thebandwidth of each of the channel signals provided by the channelizer.

More particularly, the wideband basestation transceiver includes areceive antenna and one or more digital tuners that provide widebanddigital signal energy to a digital channelizer. The digital channelizer,in turn, produces a plurality of channel signals, with each channelsignal representing the signal energy in one of the radio frequencychannels. The channel signals each consist of a series of digitalsamples.

The digital samples of each channel signal are, in turn, connected to atime division multiplex (TDM) bus. A basestation controller grantsaccess to the TDM bus by each channel signal in a predeterminedtimeslot, in a predetermined order.

The samples of the digital channel signals are then forwarded to anavailable one of the associated receiver resources, such as ademodulator. The demodulators, typically implemented in a digital signalprocessor (DSP), are then connected to an outgoing landline such as a T1line to a telephone switching office (MTSO) or mobile switching center(MSC) for further connection into the PSTN.

Accordingly, when the basestation is to support a particular number ofchannel signals simultaneously, each channel signal having a bandwidthas dictated by a particular modulation or air interface standard to besupported, the TDM bus reconfigured accordingly, to provide the requirednumber of bus time slots, as well as the frame repeat rate required tosupport the desired per-channel sampling rate.

When it is desired that the basestation support a protocol which has alarger number of narrower bandwidth channels, the bus timing circuitsare reconfigured to provide a correpondingly larger number of time slotsat a slower frame rate, and, likewise, when the basestation is tosupport a protocol which has a smaller number of wider bandwidthchannels, the bus timing circuits are again reconfigured to provide asmaller numnber of time slots at a higher frame rate.

As a result of the switching functionality provided by the TDM bus, thebasestation is thus capable of supporting different signallingprotocols, or air interface standards, which have different channelbandwidths.

The invention provides other advantages as well.

For example, the basestation may efficiently service both code divisionmultiple access (CDMA) and time division multiple access (TDMA) signalsat the same time. In such an arrangement, there are at least two digitalchannelizers, with one allocated to separating the incoming RF energyinto the channel bandwidths required by TDMA, and another channelizerdedicated to separating the energy into the bandwidth required by CDMA.As the channels are activated, they are then serviced by the pool ofdemodulator resources, by allocating the correct number of additionaltime slots to accommodate each standard.

If, for example, a wideband CDMA mobile unit goes off line, thetimeslots as modulators and demodulators freed thereby can be allocatedto processing TDMA signals. This results in automatic on-demandredistribution of basestation resources to one signaling standard oranother, without intervention by an MTSO, MSC, or the service providerin any way.

Such a system architecture also exhibits scalability, in the sense thatadditional DSP processors may be added to support additional channels astraffic increases, without having to change the RF front configuration.This is unlike the prior art, where each basestation had a fixed channelallocation, and, to add capacity, one must add additional narrowbandreceivers and transmitters.

As a result, a basestation according to the invention permits a wirelessservice provider much greater flexibility in planning implementations,as different and even future protocols can be supported.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the advantages provided by the invention,reference should be had to the following detailed description togetherwith the accompanying drawings, in which:

FIG. 1 is a block diagram of wideband digital basestation making use ofa time division multiplex (TDM) bus according to the invention;

FIG. 2 is a more detailed block diagram showing addressable bus driversand receivers which permits access to the TDM bus;

FIG. 3 is a detailed diagram of an addressable bus driver using adual-port random access memory (DP-RAM);

FIGS. 4A and 4B are timing diagrams showing the frame length and numberof time slots on the TDM bus for two different channel bandwidths;

FIG. 5 is a detailed diagram of an addressable bus driver using afirst-in, first-out (FIFO) memory;

FIG. 6 is a detailed diagram of an addressable bus receiver using aFIFO;

FIG. 7 is a detailed diagram of an addressable bus transmitter using aFIFO;

FIG. 8 is a sequence of operations performed by a basestation controlprocessor in setting up a connection; and

FIG. 9 is an alternate embodiment of the invention making use ofmultiple tuners and channelizers to support multiple air interfacestandards while making maximum use of basestation resources.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 is a block diagram of a wideband wireless digital basestation 10according to the invention. Briefly, the basestation 10 consists of areceive antenna 11, one or more wideband digital tuners 12, one or moredigital channelizers 14, a time division multiplex (TDM) bus 16, acontrol bus 17, a plurality of digital signal processors (DSPs), a firstsubset of which are programmed to operate as demodulators 18-1-1,18-1-2, . . . , 18-1-P (collectively, demodulators 18-1); a secondsubset of which are programmed to operate as modulators 18-2-1, 18-2-2,. . . , 18-2-Q; and a third subset 18-u of which are presently idle,transport signal (T-1) encoder 20, a T-1 decoder 22, one or more digitalcombiners 24, one or more wideband digital exciters 26, a poweramplifier 28, a transmit antenna 29, a basestation control processor(controller) 30, and a TDM synchronization clock generator 32.

More particularly, the basestation exchanges radio frequency (RF)signals with a number of mobile subscriber terminals (mobiles) 40a, 40b.The RF carrier signals are modulated with voice and/or data (channel)signals which are to be coupled to the public switched telephone network(PSTN) by the basestation 10. The particular modulation in used may beany one of a number of different wireless (air interface) standards suchas the well known Advanced Mobile Phone Service (AMPS), time divisionmultiple access (TDMA) such as IS-54B, code division multiple access(CDMA) such as IS-95, frequency hopping standards such as the EuropeanGroupe Speciale Mobile (GSM), personal communication network (PCN)standards, and the like. Indeed, in a manner that will be describedbelow, the basestation 10 may even be configured to simultaneouslyprocess RF signals formatted according to more than one such airinterface at the same time.

On the receive side (that is, with respect to the basestation 10), RFmodulated signals are first received at the receive antenna 11, andforwarded to the wideband digital tuner 12. The digital tuner 12downconverts the RF signal received at the antenna to a intermediatefrequency (IF) and then performs an analog to digital (A/D) conversionto produce a digital composite signal 13.

Digital tuner 12 is wideband in the sense that it covers a substantialportion of the bandwidth available to the wireless service provider whois operating the basestation 10. For example, if the air interfaceimplemented by the basestation 10 is IS-54B, the wideband digital tunermay downconvert as much as a 12.5 MegaHertz (MHz) bandwidth in the800-900 MHz range which contains as many as 416 receive and transmitchannel signals, each having an approximately 30 kiloHertz (kHz)bandwidth.

The digital channelizer 14 implements a channel bank to separate thedownconverted composite digital signal 13 to a plurality, N, of digitalchannel signals 15.

This digital sampled signal is then further filtered to separate it intothe individual 30 kHz channel signals. The digital channelizer 14 canthus be considered as a bank of digital filters with each filter havinga 30 kHz bandwidth. The digital channelizer 14 may implement the filterbank using any of several different filter structures, and no particulardigital filter structure is critical to the operation of the invention.However, in one preferred embodiment, the digital channelizer 14consists of a set of convolutional digital filters and a Fast FourierTransform (FFT) processor. The convolutional digital filters make use ofmultirate digital filter techniques, such as overlap and add, orpolyphase, to efficiently implement a digital filter bank by groupingsamples of the downconverted signal together, multiplying the samplegroups by a convolutional function, and then forwarding the samples tothe FFT for conversion into the n individual channel signals. Suchfilter banks may implemented using any of the techniques as aredescribed in the textbook by Crochiere, R. E., and Rabiner, L. R.,entitled "Multirate Digital Signal Processing" (Englewood Cliffs, N.J.:Prentice-Hall, 1983), pages 289-399.

In any event, the channelizer 14 provides N individual digital channelsignals 15, wherein each of the N outputs represent information in oneof the radio frequency channels originated by the mobile 40. Usually,one-half of the channels are used for transmitting signals and one-halffor receiving signals. Thus, in the IS-54B example being described, N is208, and thus there are 208 receive and 208 transmit channelsimplemented by the basestation 10.

These N digital channel signals are then provided over the time divisionmultiplex (TDM) bus 16 to a plurality of digital signal processors(DSPs) 18-1-1, 18-1-2, . . . , 18-1-P (collectively, demodulator-DSP18-1). In a manner that will be understood in greater detail shortly,the TDM bus 16 operates as a time division multiplexed cross-bar switch.That is, any one of the N digital channel signals 15 may be connected toany one of the demodulator DSPs 18-1 via the TDM bus 16.

The exact nature of the timing of the TDM bus 16, that is, the number oftime slots available for eaech frame of data samples output by thedigital channelizer 14, and thus the manner in which the N digitalchannel signals are transferred over the TDM bus 16, changes dependingupon the number of channel signals, N. The manner in which thebasestation 10 accomodates these changes in the timing of the TDM bus 16will be decribed in greater detail below.

The DSPs 18-1 are each programmed to remove the modulation on eachchannel signal 15 as specified by the air interface standard supportedby the basestation 10. There typically is not a one-to-onecorrespondence between the number of DSPs 18-1 and the number of channelsignals, N, provided by the channelizer 14. For example, the DSPs mayeach process a number, such as 24, of digital channel signals 15 at thesame time.

The basestation controller 30, using the VME bus and TDM synchronizationclock generator 32, manages access by individual digital channel signals15 to the TDM bus 16, in a manner that will be described shortly.

The outputs of the digital signal processors 18-1, representingdemodulated audio or data signals, are then forwarded over the VME bus17 to the encoder 20. The VME bus 17 is a well known industry standardrelatively high frequency bus for interconnecting digital processors andcomponents.

The encoder 20, in turn, reformats the demodulated signals as necessaryfor transmission to a local Mobile Telephone Switching Office (MTSO).The demodulated signals may be reformatted according to any one of anumber of well-known time multiplex telephone signal transportprotocols, such as the so-called T1 span (or E1). The T1 signals arethen processed by the MTSO in an known fashion, to ultimately complete atelephone call from the subscriber unit 40 to a desired destination,such as another telephone subscriber who is connected to the PublicSwitched Telephone Network (PSTN).

Since each T1 span has a limited capacity, there may be more than one T1signal necessary to accommodate all of the channels serviced by thebasestation 10. In the example being discussed, each T1 signal may beformatted to carry up to 96 IS-54B bandwidth-compressed signals to theMTSO, assuming that the demodulated signals remain as compressed audio.Thus, as few as five T1 lines can be used to carry all of the 416transmit and receive channels. When not all of the channels are busy,however, on as many of the T1 line resources as are necessary areconnected to the MTSO, in a manner that will be understood shortly.

In other words, the demodulated signals output by the DSPs 18-1 may eachbe sub-rate (e.g., sub-DS0 frequency signals) which still containadditional encoding other than the air interface standard, such asimpressed by a bandwidth compression scheme, which is not removed by thebasestation 10. Rather, to minimize the required number of time slotsused by the T1 signals, such compression may be removed at the MTSO.

The signal flow on the transmit side of the basestation 10 is analogous.Signals are received from the MTSO and provided to the T1 decoder 22,which removes the T1 formatting.

The unformatted T1 signals are then coupled to the DSPs 18 over the bus17. A subset of the DSPs 18-2-1, 18-2-2, . . . , 18-2-Q (collectively,modulators 18-2) then modulate these signals and presents them to theTDM bus 16. Ultimately, these are then each coupled to one of the Ndigital channel signals 23 input to the combiner 24. As was true in thereceive direction, being a cross-bar swtich, the TDM bus 16 permits anyone of the modulator DSPs 18-2 to be connected to any one of the channelsignal inputs 23.

Although each modulator DSP 18-2 typically processes multiple channelsignals, each such channel signal generated by the modulator DSP 18-2 istypically assigned one or more unique time slots on the TDM bus 16, withno two channel signals thus occupying the same time slot. Similarly, notwo channel signals on the receive side ever occupy the same timeslot onthe TDM bus 16.

As for the demodulators DSPs 18-1, the number of time slots assigned perframe on the TDM bus 16 varies, depending upon the channel bandwidth ofthe moduation standard implemented.

Other DSPs 18-u may be unused at a particular point in time. However,these unused DSPs remain as an available resource to the basestation 10,should a new mobile 40 request access. The manner in which DSPs areallocated at the time of setting up a call will be described in detailbelow.

The digital combiner 24 combines the TDM bus outputs to produce acomposite IF digital signal 25 representing the N channels to betransmitted. The digital combiner 24 then feeds this combined signal toa digital exciter 26, which generates an RF signal 27. This RF signal 27is then amplified by the power amplifier 28 and fed to the transmitantenna 29.

In order to set up each call, the basestation control processor 32 mustexchange certain control information with the MTSO. For example, when amobile unit 40 wishes to place a call, the mobile unit 40 indicates thisby transmitting on one or more control signal channels. These controlsignals may be exchanged in one of several ways. As shown, the controlsignals may be in-band or out of band signals present in one or more ofthe channel signals output by the channelizer 14 or input to thecombiner 24. Alternatively, a separate control signal transceiver 35 maybe used to receive and transmit such control signaling.

In either event, the basestation 10 forwards the request for access bythe mobile 40 to the MTSO, to set up the end to end connection. Uponreceiving an indication from the MTSO that the connection can be made atthe remote end, the basestation 10 then performs a number of steps, toinsure that the appropriate data path through the TDM bus is thenenabled to support communication with between the newly enabled mobile40 and the MTSO.

For example, the MTSO typically returns a pair of T1 span line and T1time slot identifiers. These inform the basestation controller 30 onwhich outgoing T1 line and time slot to place the received signal, andon which incoming T1 line and time slot it can expect to obtain thetransmit signal for the mobile 40.

However, before proceeding with a detailed explanation of this callset-up process, a bit more detail of the operation of the TDM bus 16will be provided. As shown in FIG. 2, the digital channelizer 14consists of a convolutional digital filter 140, a fast Fourier transform(FFT) 142, as well as a TDM dual port (DP) driver 144.

The operation of the convolutional filter 140 and FFT 142 is notcritical to the present invention, and is explained in the co-pendingapplication. It is sufficient here to say that the convolutional filter140 and FFT 142 make use of multirate digital signal processingtechniques, such as overlap and add or polyphase, to efficientlyimplement a digital filter bank by (1) grouping samples of thedownconverted signal 13 together and multiplying them by a weightingfunction, and then (2) forwarding them to the FFT 142 for conversioninto the N individual channel signals.

An exemplary DSP demodulator 18-1-1 and modulator 18-2-1 are also shownin FIG. 2. The demodulator DSP 18-1-1 includes a TDM first-in first-out(FIFO) driver 180-1, a TDM FIFO receiver 182-1, a DSP central processingunit 184-1 and program memory 186-1. Similarly, the modulator DSP 18-2-1includes a TDM FIFO driver 180-2, a TDM FIFO receiver 182-2, a DSPcentral processing unit 184-2 and program memory 186-2.

Indeed, the modulator and demodulator DSPs may share the same hardwarearchitecture, with the only difference being the in the program which isenable in the program memory 186, which in turn may control whether theTDM receiver or TDM driver hardware is enabled.

Thus, in the DSP demodulator 18-1-1, only the TDM receiver 182-isenabled (as indicated by the dashed lines around the driver 180-1),since the demodulator 18-1-1 only receives data from the TDM bus 16.Likewise, only the TDM driver 180-2 is enabled in the DSP modulator18-2-1, since it only transmits data on the TDM bus 16.

On the transmit side, the digital combiner 24 consists of a TDM dualport (DP) receiver 244, an inverse FFT 242, and deconvolutional digitalfilter 240. In a manner that is described below, the TDM DP receiver 244reads each of the data samples off the TDM bus 16 in their assigned timeslot, and provides them to the inverse FFT 242 in the required order.

The samples are then operated on by the inverse FFT 242 anddeconvolutional filter 240, to provide the composite digital signal 25(FIG. 1).

Returning attention now to the channelizer 14, a detailed diagram of theTDM DP driver 144 is shown in FIG. 3. Briefly, it operates to assert theoutput samples from the FFT 142 in the proper time slots on the TDM bus16. In order to simplify the implementation of the TDM bus 16, thesetime slots are fixedly assigned to particular channels (such as inascending order by frequency and time slot number). Thus, a sample of agiven one, k, of the N channel signals, will always appear in aparticular time slot, k, when it is active.

The DP driver 144 consists of a TDM slot counter 200, a first Dual PortRandom Access Memory (DP-RAM) referred to as the enable DP-RAM 202, asecond DP-RAM referred to as the data DP-RAM 204, and a driver 208having an enable input EN.

As is conventional, each of the DP-RAMs have two separate address anddata ports for reading and writing data, namely, input address and dataports AI and DI, and output address and data ports AO and DO.

In operation, the TDM slot counter 200 receives a pair of signalsgenerated by the TDM synchronization circuit 32 (FIG. 1). The firstsignal, TDM CLK, is a digital clock signal identifying the clockperiods, or time slots, on the TDM bus 16. The second signal is a TDMFRAME SYNC signal, indicating when a new frame starts on the TDM bus 16.

The TDM slot counter 200, which is a standard digital counter, receivesthe TDM FRAME SYNC signal at a reset input R, and the TDM CLK signal ata clock input (denoted by a chevron in the Figures). Thus, the TDM slotcounter 200 continuously keeps track of which consecutively numberedslot on the TDM bus 16 is presently active.

According to the invention, the manner in which the signals aremultiplexed onto the TDM bus 16 is changed, depending upon the bandwidthof channels in the modulation scheme being supported. In particular, thenumber of time slots per frame on the TDM bus 16 is adjusted, dependingupon the bandwidth of the modulation of the air interface which isimplemented.

Thus, for different air interface standards, the TDM slot counter 200will receive different TDM CLK and different TDM FRAME SYNC signals.Turning attention briefly to FIGS. 4A and 4B, this concept will bebetter understood. As shown in FIG. 4A, for the IS-54B TDMA standard,the channelizer 14 provides 208 channels, each having a 30 kHzbandwidth. The desired complex-valued (e.g., in-phase and quadrature)sampling rate of each TDMA channel is approximately 40 kHz, so that theframe rate, that is, the rate at which each group of 208 samples isasserted on the TDM bus 16, is also set to 40 kHz.

Accordingly, in order to support IS-54B channels, the TDM FRAME SYNCsignal is controlled by the TDM synchronization clock generator 32 toreset the TDM slot counter 200 every 1/40 kHz, or every 25 μs, and theTDM CLK signal is set to clock the complex-valued samples, one from eachof the 208 channels, every 25 μs/208; in other words, to provideapproximately one TDM time slot every 121 ns.

As shown in FIG. 4B, for the IS-95 CDMA standard, the channelizerprovides 10 channels, each having a 1.25 Mhz bandwidth. The desiredcomplex-valued sampling rate of each channel is approximately 1.67 MHz,so that the frame rate is 600 ns.

Accordingly, in order to support IS-95 channels, the TDM FRAME SYNCsignal is controlled to reset the TDM slot counter 200 every 600 ns, andthe TDM CLK signal is set to clock the complex-valued samples, one fromeach of the 10 channels, every 600 ns/10, or to provide approximatelyone TDM time slot every 60 ns.

As shown in FIG. 1, the TDM synchronization clock generator receivesappropriate signals from the basestation controller 30 via the VMS bus17 indicating the desired TDM CLK and desired TDM FRAME SYNC rate.

The manner in which data may be asserted on the TDM bus 16 in any of thetime slots will now be described in detail. In particular, the enableRAM 202 generates an enable signal 203 indicating when the TDM DP driver144 may assert data on the TDM bus 16. The AI and DI inputs to theenable DP-RAM 202 are typically written into by the basestationcontroller 30 during the process of setting up a new call. Inparticular, as shown in the table depicting the contents of the enableDP-RAM 202, a location in the RAM is associated with each time slot onthe TDM bus 16 (e.g., if the TDM bus contained 512 time slots, then theRAM 202 has 512 locations).

A logical "0" in the associated enable DP-RAM 202 location indicatesthat the TDM driver is inactive in the time slot, that is, no data is tobe asserted at that time. A logical "1" in the associated locationindicates that the time slot has been assigned to this particular TDMdriver 144.

Thus, to enable a connection through the TDM bus 16, one step for thebasestation controller 30, via the VME bus 17, is to write a logical "1"into the DP-RAM 202 location "x" associated with the newly enableddigital channel signal "x". In the example, shown, a "1" has beenwritten at locations "27" and "30", indicating that this particular TDMdriver 144 is now active in timeslot numbers 27 and 30.

The data DP-RAM 204 acts as a buffer, writing the digital channel signalsamples output by the FFT at the DI input of the data DP-RAM 204. TheDP-RAM 204 then stores the data samples until addressed by the TDM slotcounter at the output side.

A data dual port (DP) RAM 204 is as a buffer in the case of processingthe FFT output. This is because although the samples do come in bursts,or frames, the samples are not necessarily provided by the FFT 142 inthe same order as they must be output onto the TDM bus 16. This is aparticular phenomenon of at least one of the channelizer algorithmsused. Thus, an address associated with each output sample from the FFTis used to determine at which location each sample is written in thedata DP-RAM 204.

However, the input data is already in the correct order for the TDM FIFOdriver 180-2 used by the DSP modulator. Such a TDM driver 180-2 can thususe a first-in first out memory (FIFO) 210 in the place of a dataDP-RAM. As shown in FIG. 5, the configuration and operation of such asTDM FIFO driver 180-2 is somewhat similar to the DP driver 144.

In particular, the TDM slot counter 200, enable DP-RAM 202 and driver208 operate in the same way as for the embodiment of FIG. 3. The onlydifference is in the connection of the clock signals to the FIFO 210. Onthe input side, a clock signal is provided by the data source (e.g., theDSP processor 184-2) to cause data to be stored in the FIFO. The signalfrom the enable DP-RAM 202 is used to clock the FIFO output, DO.

A detailed diagram of the TDM FIFO receiver 182-1 is shown in FIG. 6. Itincludes a TDM slot counter 200, enable DP-RAM 202, bus receiver 212,and FIFO 214. The TDM slot counter 200 and enable DP-RAM 202 operate asfor the TDM FIFO driver 180-1 shown in FIG. 5, to identify when thereceiver 212 is to be active. The FIFO 214 is connected to the output ofthe receiver 212, having its input port connected to the enable DP-RAM202 output. The output side of the FIFO is clocked as needed by thedestination for the data (such as the DSP processor 184-1 in FIG. 2).

The TDM DP receiver 244 is shown in detail in FIG. 7. As for each of theother driver/receivers, it includes a TDM slot counter 200, enableDP-RAM 202. It includes a data DP-RAM 220 operating similarly to thedata DP-RAM in the TDM DP driver 144 (FIG. 3) and bus receiver 218.

With this background in mind, the details of how the basestation controlprocessor 30 effects the switching operation of the TDM bus 16 can nowbe better understood.

FIG. 8 is a flowchart of these operations. This sequence of steps isinitiated (step 300) when the basestation controller 30 receives controlsignals from the mobile 40 (FIG. 1) indicating that the mobile wishes tohave access to the PSTN. The controller 30 then determines whether afree transmit and receive frequency (step 302) are available among the Nchannels.

An available modulator DSP and demodulator DSP resource are thenidentified (step 303) by examining a list 33 of free DSP resourcesmaintained in a memory portion 31 of the basestation controller 30 (FIG.1). The list 33 is updated by removing the two DSPs once allocated.

Access to an MTSO T1 channel (e.g., access to one or more T1 time slotsas needed on a particular T1 span line) is then requested from the MTSOby issuing the appropriate MTSO control signaling (step 304). The MTSOthen returns T1 span and time slot identifiers to be used for thetransmit and receive channels for this connection.

In the next step (306), the appropriate destination and sourceinformation is written into the various TDM bus drivers and receivers.

In particular, given a receive channel identification, a receive channelsignal time slot on the TDM bus is thus identified. The correspondinglocation of the enable DP-RAM 202 in the TDM DP driver 144 associatedwith this time slot is then set to a logical "1" in the manner alreadydescribed.

Next, a logical "1" is also written into the enable DP-RAM in the TDMreceiver 182-1 associated with the DSP demodulator 18-1 which wasidentified as being an available resource. If the per-channel bandwidthis greater than that which can be supported by a single timeslot, then asufficient number of logical "1"s are written into the appropriatelocations.

Also, now given a transmit channel identification, the free DSPmodulator 18-2 is enabled (step 306) to use the TDM bus 16, by writing alogical "1" into the enable DP-RAM of the TDM driver 180-2 connected tothe available one of the DSP modulators 18-2. To complete theconnection, a logical "1" is also written into the location of the TDMDP receiver 244 associated with the identified transmit channel.

Finally (step 308), the basestation controller 30 issues control signalsto the mobile 40 and MTSO to indicate that the connection has been setup.

The invention can also be used to advantage in implementing abasestation 10 which simultaneously services mobiles 40 which usedifferent air interface standards. That is, the basestation 10 may atthe same time process signals from a first mobile 40a which uses TDMA(IS-54B) signaling, as well as a second mobile 40b which uses CDMAsignaling (IS-95).

As shown in FIG. 9, to support this implementation, the basestation 10includes a pair of wideband digital tuners 12-1, 12-2. The first digitaltuner 12-1 downconverts a bandwidth, such as 5 MHz, from an RF bandwidthwhich is occupied by TDMA signals. A second digital tuner section 12-2downconverters a bandwidth, such as 7.5 MHz, which is occupied by CDMAsignals.

Next, the tuners 12-1, 12-2 forward the downconverted signals torespective channelizers 14-1, 14-2. The TDMA channelizer 14-1 isconfigured to separate the received signal into the 30 kHz bandwidthchannels specified by IS-54B. Likewise, the CDMA IS-95 channelizer 14-2is configured to provide 1.25 MHz channels as specified by thatstandard.

The TDM synchronization clock generator 32 is appropriately set toprovide a sufficient number of time slots on the TDM bus 16 to permittransmission of both the required number of samples from the TDMAchannelizer 14-1 as well as the required number of samples from the CDMAchannelizer 14-2.

The modulators and demodulators are then grouped according to the airinterface modulation they must deal with. For example, at any giveninstant in time, a certain number of DSPs 18-1-T will have beenallocated to operate as demodulators for the TDMA channels provided bythe TDMA channelizer 14-1. A different set of DSP processors 18-1-C willbe serving as demodulators for the CDMA channels provided by the CDMAchannelizer 14-2. The active modulator DSPs will likewise be soallocated.

Thus, assuming that each of the DSPs 18 can be configured to executeeither a TDMA modulation/demodulation program or a CDMAmodulation/demodulation program by simply accessing the correct programmemory, the available DSP resources and TDM bus time slots are onlyallocated as needed.

In other words, the DSPs (and associated T1 connections, for thatmatter) are allocated according to user demand automatically, andwithout intervention by the service provider. Thus, for example, as morecustomers migrate to using CDMA, additional CDMA channels areautomatically made available and processed by the DSPs, at the expenseof the unused TDMA channels.

A number of advantages can now be see for a basestation 10 configuredaccording to the invention.

When the basestation is to receive and transmit channel signals having abandwidth as dictated by a particular modulation or air interfacestandard to, the TDM bus is reconfigured accordingly, to provide therequired number of bus time slots, as well as to provide a frame repeatrate approraite to support the required per-channel sampling rate.

When, at a different time, the basestation is to support a protocolhaving a larger number of narrower bandwidth channels, the bus timingcircuits are reconfigured to provide a correpondingly larger number oftime slots at a slower frame rate. Likewise, when the basestation is tosupport a protocol which has a smaller number of wider bandwidthchannels, the bus timing circuits are again reconfigured to provide asmaller numnber of time slots at the higher required frame rate.

By disposing the TDM bus 16 between the output of the wideband digitalchannelizer 14 and the demodulator DSPs 18-1, the demodulator DSPs 18-1may be allocated only as needed. Similarly, the modulator DSPs 18-2 areallocable as needed, since the TDM bus 16 is disposed between them andthe digital combiner 24 as well.

Thus, if the basestation 10 is expected to service only a small numberof channels, a correspondingly small number of modulator and demodulatorDSPs can be installed in the basestation 10. As the basestation'sdemands increases, these additional RF channels can be serviced bysimply adding more DSPs, and without having to reconfigure the RF frontend.

Another advantage is provided in that this switching functionality isdistributed at the basestation level as much as possible. In particular,unlike certain prior cellular signal switching techniques, the MTSO neednot be concerned with the details of how the mobile units 40 areconnected through the basestation. Indeed, the MTSO need not even knowor care about which transmit and receive frequencies have been assignedto a particular mobile. All the MTSO need provide is identification of aT1 transport line and time slot on which it expects to receive andprovide signals from and to the mobile.

Furthermore, because the basestation may efficiently allocate itsdemodulator/modulator resources, a number of different air interfacestandards may be supported by the basestation at the same time, withoutthe need to determine in advance an exact plan for allocatingreceiver/transmitter resources for each air interface type. Upondetecting a request by a new mobile for access, the basestation simplydetermines the type of air interface used by the mobile, and thensignals the appropriately programmed DSPs, or even initiates the DSPs torun a different modulator/demodulator program, as required to supportthe additional mobile.

While we have shown and described several embodiments in accordance withthe present invention, it is to be understood that the invention is notlimited thereto, but is susceptible to numerous changes andmodifications as known to a person skilled in the art, and we thereforedo not wish to be limited to the details shown and described herein butintend to cover all such changes and modifications as are obvious to oneof ordinary skill in the art.

What is claimed is:
 1. A basestation for processing signals in amultiple mobile subscriber unit wireless communication systemcomprising:means for converting signals, in the form of a compositeradio frequency, RF, signal sent from a plurality of the mobile units,into a first set of multiple digital channel signal outputs, each saiddigital channel signal output corresponding to one of the signalsreceived from one of the plurality of mobile units; means for selectinga modulation scheme supported by the basestation at a specific time: aplurality of digital signal processing means, for providing digitallyprocessed channel signal outputs according to said selected modulationscheme being supported by the basestation at a specific time; timedivision multiplex switching means, disposed between the means forconverting signals and the plurality of digital signal processing means,the switching means for interconnecting any one of the multiple digitalchannel signal outputs to any one of the plurality of digital signalprocessing means; and time division multiplex synchronization timingmeans, coupled to control the time division multiplex means, to adjustand change a frame rate and a number of time slots per frame, dependingupon a bandwidth of the selected modulation scheme being supported bythe basestation.
 2. The basestation according to claim 1, wherein themeans for converting signals further includes:wideband digital tunermeans, for downconverting a selected bandwidth of the composite radiofrequency, RF, signal to an intermediate frequency, IF, signal, and forperforming an analog to digital conversion on the IF signal, to providea wideband digital tuner output signal; and digital channelizationmeans, coupled to receive the wideband tuner output signal, forproviding the first set of multiple digital channel signal outputs, thedigital channelization means being adjustable such that each digitalchannel signal output to the time division multiplex switching means hasa predetermined channel bandwidth, such that each digital channel signaloutput is output during a corresponding time slot associated with thatdigital signal channel output onto the time division multiplex switchingmeans.
 3. The basestation according to claim 2, wherein the digitalchannelization means is controlled by outputs from the time divisionmultiplex synchronization timing means, at least one output identifyingtime slots occurring on the time division multiplex switching means suchthat there is one time slot per digital channel signal output, and atleast one other output indicating when a new frame starts on the timedivision multiplex switching means.
 4. The basestation of claim 1wherein the signals received from the mobile units contain modulation,and the digital signal processor means include demodulators to removethe modulation.
 5. The basestation of claim 1 additionally comprisingsignal-transport encoding means, connected to the output of the digitalsignal processing means, for encoding the digitally processed channelsignal outputs for further transmission to a mobile telephone switchingoffice, MTSO.
 6. The basestation of claim 5 wherein the signal-transportencoding means is a T1 encoder.
 7. The basestation of claim 2additionally comprising:second digital channelization means, beingconnected to receive the wideband tuner output signal, and providing asecond set of multiple digital channel signal outputs, each one of thesecond set of the digital channel signal outputs having a predeterminedchannel bandwidth which is different from the predetermined channelbandwidth of said first set of digital channel signal outputs, and eachone of the second set of multiple digital channel signal outputscorresponding to one of the signals received from one of the mobileunits.
 8. The basestation of claim 2 additionally comprising:secondwideband digital tuner means, for downconverting a second selectedbandwidth of the composite radio frequency RF signal to a secondintermediate frequency, IF, signal, and for performing an analog todigital conversion on the second IF signal, to provide a second widebanddigital tuner output signal; and second digital channelization means,coupled to receive the second wideband digital tuner output signal, andproviding a second set of multiple digital channel signal outputs, eachone of the second set of the digital channel signal outputs having apredetermined channel bandwidth which is different from thepredetermined channel bandwidth of said above mentioned first set ofdigital channel signal outputs, and each one of the second set ofdigital channel signals corresponding to one of the signals receivedfrom one of the mobile units.
 9. A basestation as in claim 1 wherein thefirst and second set of digital channel signal outputs are modulated inaccordance with first and second air interface standards, respectively.10. A basestation as in claim 9 wherein the digital signal processorsinclude a first set of digital signal processor means for demodulatingsaid first set of digital channel signal outputs, and a second set ofdigital signal processors for demodulating said second set of digitalchannel signal outputs.
 11. A basestation as in claim 9 wherein saidfirst and second air interface standards are each a different one of atime division multiplex, TDMA, code division multiplex, CDMA, AdvancedMobile Phone System, AMPS, Personal Communications System , PCS, orGroupe Especiale Mobile, GSM.
 12. A basestation as in claim 1additionally comprising:basestation controller means, connected to thetime division multiplex switching means and the digital signalprocessing means, for maintaining a list of unused digital signalprocessing means that are not presently interconnected through the timedivision multiplex switching means to one of the digital channel signaloutputs, and for dynamically allocating digital signal processing meansfrom the list of unused digital signal processing means to beinterconnected to one of the digital channel signal outputs only whenthe digital channel output contains an active signal being transmittedby the mobile unit which has not yet been assigned to one of the digitalsignal processing means.
 13. A basestation as in claim 1 wherein thetime division multiplex switching means further comprises:a timedivision multiplex, TDM, data bus including data lines; basestationcontroller means, connected to the TDM bus, and to generate TDM bussynchronization signals and driver address signals, the TDM bussynchronization signals used to identify access timeslots on the TDMbus; and TDM bus driver means, connected to the TDM bus, the basestationcontroller means, and at least one of the digital channel outputsignals, for receiving the TDM bus synchronization signals and thedriver address signals, for storing the driver address signals, and forasserting the at least one digital channel output signal on the TDM buswhen the value of driver address signals corresponds to the value of thebus synchronization signals, thereby indicating that a timeslotassociated with the at least one digital channel output signal iscurrently active.
 14. A basestation as in claim 13 wherein thebasestation controller means, connected to the TDM bus, additionallygenerates receiver address signals, and the time division multiplexswitching means additionally includes:TDM bus receiver means, connectedto the TDM bus, the basestation controller means, and at least one ofthe digital signal processor means, for receiving the TDM bussynchronization signals and the receiver address signals, for storingthe receiver address signals, and for reading a signal asserted on theTDM bus and providing such asserted signal to the digital signalprocessor means when the value of receiver address signals correspondsto the value of the bus synchronization signals, indicating that atimeslot associated with the digital signal processor is currentlyactive.
 15. A basestation as in claim 8 wherein the first and secondchannelizers each comprise:a convolutional digital filter, connected toreceive the respective one of the digitized wideband signals; and a fastFourier transform, FFT, processor, connected to receive the output ofthe convolutional digital filter, and to provide the digital channelsignal outputs.
 16. A basestation as in claim 1, further comprising:asecond plurality of digital signal processing means, connected toreceive digital input signals from a communication signal source; awideband digital combiner, being connected to receive a second pluralityof digital channel signals, and to provide a composite digital signalfor transmission; wherein the time division multiplex switching means isalso disposed between the second plurality of digital signal processorsand the wideband digital combiner, the switching means connecting anyone of the second set of digital signal processors to any one of thedigital channel signals input to the combiner; a wideband digitalexciter, connected to receive the composite digital signal and toprovide a combined RF signal; and a transmit antenna, connected toreceive the combined RF signal and the radiate the combined RF signal.17. A basestation as in claim 16, additionally comprising:basestationcontroller means, connected to the time division multiplex switchingmeans and the first and second plurality of digital signal processingmeans, for maintaining a list of all unused digital signal processingmeans that are not presently interconnected through the time divisionmultiplex switching means to one of the digital channel signal outputs,and for dynamically allocating digital signal processing means from thelist of unused digital signal processing means to function as one of thefirst or second digital signal processing means only when the digitalchannel signal output contains an active signal being transmitted by themobile unit which has not yet been assigned to one of the digital signalprocessing means, or when the digital input signals from thecommunications source are active.
 18. A basestation for processingsignals in a multiple mobile subscriber unit wireless communicationsystem, comprising:means for selecting a modulation scheme supported bythe basestation at a specific time; digital channelization means,connected to receive a composite signal containing signals received froma plurality of mobile subscriber units, for filtering the compositesignal to generate a plurality of digital channel signal outputs inresponse thereto, the digital channelization means having a selectablechannel bandwidth for each such digital channel signal output dependingupon an air interface standard being supported by the basestation; aplurality of digital signal processing means, for providing digitallyprocessed channel signal outputs according to said selected modulationscheme being supported by the basestation at said specific time; timedivision multiplex switching and synchronization means, disposed betweenthe digital channelization means and the plurality of digital signalprocessing means and interconnecting any one of the plurality of digitalchannel signal outputs to any one of the plurality of digital signalprocessing means; wherein the digital channelization means and the timedivision multiplex switching and synchronization means are coupled suchthat each is driven by common parameters including a frame rate and anumber of time slots per frame, the common parameters determining whichdigital channel signal is interconnected between the digitalchannelization means and a digital signal processing means at any onemoment in time, and said parameters being determined by the airinterface standard being supported by the basestation.
 19. Thebasestation according to claim 18, wherein:the time division multiplexswitching and synchronization means includes:a time division multiplexsynchronization means; and a time division multiplex switching means;and wherein the common parameters are produced by a time divisionmultiplex synchronization means and synchronize the digitalchannelization means and the time division multiplex switching meanssuch that the digital signal channel outputs are multiplexed onto thetime division multiplex switching means at a predetermined rate,depending upon the bandwidth of channels allocated in the modulationscheme being supported; and wherein the common parameters may be changedto allow the basestation to implement reception and transmission of avarying number of signals having varying bandwidth requirements,depending upon different air interface standards being supported by thebasestation.
 20. The basestation according to claim 19, wherein thecommon parameters may be adjusted to reconfigure the basestation tosupport at least two different air interface standards.
 21. Thebasestation according to claim 20, wherein said at least two differentair interface standards include at least one selected from the groupcomprising the IS-54B TDMA standard, the IS-95 CDMA standard, anAdvanced Mobile Phone System, AMPS, standard, a Personal CommunicationsSystem, PCS, standard, and a Groupe Especiale Mobile, GSM, standard.